Ion source for a mass spectrometer with specific electrode structure to accelerate and focus ions



prl 23, 1968 c. w. BAKER 3,379,874

vION SOURCE FOR A MASS SPECTROMETER WITH SPECIFIC ELECTRODE STRUCTURE ToACCELERATE AND FOCUS TONS Filed June 29, 1964 2 Sheets-Sheet l April 23,1968 c. w. BAKER 3,379,374

ION SOURCE FOR A MASS SPECTROMETEH WITH SPECIFIC ELECTRODE STRUCTURE TOACCELERATE AND FOCUS IONS Filed June 29, 1964 2 Sheets-Sheet 2 UnitedStates Patent O 3,379,874 ION SOURCE FOR A MASS SPECTROMETER WITHSPECIFIC ELECTRODE STRUCTURE TO AC- CELERATE AND FOCUS IONS ClarenceWilliam Baker, Monrovia, Calif., assigner to Bell & Howell Company,Chicago, lll., a corporation of Illinois Filed June 29, 1964, Ser. No.378,910 4 Claims. (Cl. Z50-41.9)

This invention relates to mass analyzers and in particular to animproved ion source for use with such analyzers.

Generally speaking, mass analyzers, as for example, mass spectrometers,consist of three parts: an ion source, an analyzing sector and acollector section. Located within the ion source in many types ofinstruments and interposed in the direction of ion travel are three ormore electrodes to which various potentials are applied in order toaccelerate the ions before they -pass into the analyzing sector, Theseelectrodes are normally thin metallic members in which apertures orslits have been cut in order that the ions may pass through them and becollimated into a thin beam or ribbon of ions before passing into theanalyzing sector.

Since there is normally a difference of potential established betweenthe first and second electrode, an electrode field exists in the spacebetween the two electrodes, and this field has been found to bedistorted adjacent the apertures in these two electrodes. It has furtherbeen found that such distortion has a significant lens effect on thedirection of acceleration of the ions. As the ions pass through theaperture in the first electrode, they are deflected toward the opticalor central axis of the analyzer. As they pass through the field adjacentthe slit in the second electrode, they are deflected away from thiscentral axis. The result is that the ion yield at the slit in the thirdelectrode is substantially reduced over that available at the slit inthe second electrode.

Other problems have also been encountered. For eX- ample, ions impingingon the sides of the slit in the second electrode are found to causesecondary emission of electrons which were attracted to and deposited onthe back of the first electrode. This current flowing from the second tothe first electrode tends to load down the accelerating voltage supplycausing a varying accelerating voltage and consequent low ion yield.

One possible means of increasing the ion yield at the third electrodeslit is to increase the width of the slit in the second electrode. Inthis way a greater number of ions pass through the slit in the secondelectrode. However, a wider angle of acceptance due to the wider slitalso produces greater distortion of the adjacent electric field andincreases ion scatter. Thus, the two effects offset each other.

The present invention increases the ion yield at the third electrode byproviding an electrode system for collimating and propelling ions as abeam outwardly from an ion source. The system comprises a firstelectrode having an aperture therein, a second electrode having a gridor array of curved conductors which is aligned with the first electrodeand displaced from it in the direction of ion propulsion. Located on theside of the second electrode opposite the first electrode is a thirdelectrode aligned with the first two, this electrode also having anaperture therein for the passage of ions. Connected to the first andsecond electrodes are means for imposing a potential between the firstand second electrode for propelling the ions.

In an alternate form of the invention, a plurality of grid-typeelectrodes are provided in the area bounded 3,379,874 Patented Apr. 23,1968 by the first and third electrodes. Increasing the number ofelectrodes has the effect of forming a more predictable electric fieldand minimizing distortion due to surface charges or nearby conductors inthe array. In still another alternate to the invention, a secondelectrode in the form of a mesh is located between the first and thirdelectrodes.

These and other features of the invention will be more clearlyunderstood by reference to the following figures where:

FIG. 1 is a schematic representation of a mass spectrometer providedwith an ion source in accordance with the prior art,

FIG. 2 is a perspective view of an ion source provided with an electrodein accordance with one form of the invention,

FIG. 3 is a plan view of the ion source depicted in FIG. 2 showing theequipotential distribution produced by an electrode provided inaccordance with the invention,

FIG. 4 is an ion source with a plurality of grids provided in the spacebetween the first and third electrodes, and

FIG. 5 is a plan view of grid electrode showing the potentialdistribution adjacent a grid electrode provided with verticalconductors.

Referring to FIG. 1, a conventional mass spectrometer 5 is shownschematically and includes an iron source 6, an analyzer section 7 and acollector section .9. Within the iron source are located sparkelectrodes 10 and 11 connected at opposite ends to the secondary Windingof a Tesla transformer 13, The midpoint of the transformer winding isconnected to the positive side of a high voltage supply 14. First,second and third electrodes 1'6, 17 and 1-8 are arranged serially,electrode 16 being connected through a suitable voltage divider 20 tothe high voltage supply 14 and serving as the first electrode in asystem for propelling ions from the spark source as a beam towardelectrodes 17 and 18. Electrode 17 has an aperture 4 defining thegeometrical width of the beam and electrode 18 has an aperture 19 actingas a limit on angular divergence of the beam passing therethrough, bothelectrodes being grounded as shown.

The analyzing sector 7 in this particular type of mass analyzercomprises an electric sector 21 constituting curved plates 22 and 23arranged serially with respect to the electrodes, and a magnetic sector28. The two plates 22 and 23 of the electric sector are connected toopposite sides of a deiiecting voltage supply 24, and, as illustrated,both plates are at a pre-assigned potential with respect to ground.

The magnetic sector is formed in the conventional manner by a pair ofmagnetic poles, one of which, pole piece 26, is shown in the drawing. Inaccordance with normal practice, a second identical pole piece (notshown) is disposed parallel to and spaced from the pole piece 26 to forma magnetic field therebetween transversely of the direction of iontravel into the magnetic field from the deiiector. The collector section9 frequently comprises a photographic plate 8 immersed in the magneticfield for detecting mass dispersion therein.

The accelerating field configuration in conventional ion sources isshown in FIG. 1 by the dotted lines 15 located in the area between thefirst and second electrodes. It will be observed that the equipotentiallines are parallel to the first and second electrodes in the middle areabetween the two electrodes. However, in the areas adjacent the apertures3 and 4 in the two electrodes the field is deformed as indicated.Adjacent the first electrode the field has a bulge or dip toward theaperture 3. Near the second electrode the field has a similardeformation in the opposite direction. Arrows 25 and 27 normal to the iequipotential lines from which they extend indicate the direction ofdeflection of ions as they encounter the field distribution at thoseparticular points. As will be discussed in more detail in connectionwith FIG. 3, the shape of the electric field has a direct bearing on thefocusing or defocusing of the ions on the various apertures or slits.

The illustration in FIG. 1 is based on a` double focusing massspectrometer, and in particular, one used for the analysis of solidmaterials. However, the invention is not limited to this particular typeof mass analyzer or ion source. A field forming electrode systemprovided in accordance with this invention is equally well adaptable toother types of ion sources, such as for example, ones in which ions arecreated by electron bombardment, thermal emission, or other means suchas are used in a wide range of analyzer types.

Referring now to FIGS. 2 and 3, there is illustrated in perspective andplan view a mass spectrometer ion source provided with a field forminggrid in accordance with this invention. In addition, coordinate axes, X,Y and Z, are

shown to facilitate description of the relations between the variouselectrodes. As in FIG. 1, the ion source is provided with a Teslatransformer 29 which is connected to spark electrodes 3f) and 3l and toa high voltage supply 33. A first electrode 37, also referred to in theart as the first aperture, is provided with a circular aperture 32 andis located adjacent the spark electrodes. By means of connection 35 thefirst electrode is also connected to the high voltage supply 33.Displaced from the first electrode in the direction of the analyzingsector is a second electrode 34, which will be referred to as the fieldforming grid, which consists of an array or grid of curved conductors 36disposed in the ion beam path. The particular configuration of the gridof electrode 34, as shown in FIGS. 2 and 3, is the preferredconfiguration according to this invention. This preferred configurationcomprises a number of curved conductors located transversely of themajor or long axis of the slit 38 in a third electrode 39, usuallyreferred to as the object slit. As illustrated in FIG. 2 theseconductors are arranged paralle! to each other and to the X-Y plane.FIG. 3 more graphically illustrates the curvature which is built intothe conductors of the field forming grid 34. As shown, the curvature ofthe conductors is in one plane only, the horizontal or X-Y plane.

To produce a curved or shaped electric field, a potential differencebetween the first aperture and the combination of field forming grid andobject slit is established,

This is accomplished by connecting the first aperture to the highvoltage supply by means of the connection 35, while the grid and objectslit are grounded. The curvature of the electric field represented byequipotential lines 4t), 42, 44, 46, 48 and 50 is caused by the aperture32 and the shape of the grid 34. Examining the equipotentialdistribution of FIG. 3 in more detail, we find that in the regionadjacent the aperture 32 the equipotential lines 4o and 42 have a bulgeor dip in the direction of the spark electrodes. This deformation of theelectric field is caused by the opening 32 in the electrode. As thespace between the first and second electrodes is traversed, the effectof the curvature of the second electrode 34 is encountered. This effectis indicated by the shape of the equipotential lines 44, 46, 4S and S0.

The curvature `of the second electrode 34 is determined by its distancefrom the aperture 38. In order to obtain maximum ion yields at theobject slit, the curvature of the field forming grid 34 in theembodiment shown in FIG. 3 is circular and is arranged such that itscenter of focus is located in the center of the slit 38, and the grid ispositioned such that its axis of symmetry lies in the same plane as theZ axis and the major axis of the slit 38. In instances where the fieldforming grid is placed such that the axis of symmetry does not coincidewith this plane, its shape must be adjusted in order that focusing isstill obtained at the center of the slit 38 if maximum ion yield isstill to be obtained.

The direction of this acceleration is indicated by arrows 52 and S4. Thereason for this behavior is that charged particles placed in an electricfield are accelerated in the direction of the highest potenial gradient,i.e., the direction of the shortest distance between adjacentequipotential lines. Arrows 52 and 54 represent th's shortest distancebetween adjacent equipotential lines 48 and Eil. Since the curvature ofthe electrode 34 determines the shape of the electric field in theregion adjacent the electrode, the curvature of equipotential lines isnearly identical to that of the electrode and hence the direction of ionacceleration coincides with the radii of curvature of the grid. In thisway the ions are focused on the center of the slit 3S.

As illustrated in FIG. 2 the second electrode 34 has a curvature in theX-Y plane only. Under these conditions focusing is also confined to theX-Y plane. However, in certain instances it may be useful to achievefocusing in two planes. This is accomplished by changing the shape andconfiguration of the second electrode 34 so that it is new in the formof a partial sphere. In this condition the grid has the capability ofnot only focusing in the X-Y plane as shown in FIG. 3, but also in theX-Z plane.

It should be noted that the preceding discussion is postulated on theuse of a grid provided with conductors which are arranged transverselyof the major axis of the object slit; in this case, horizontally. Thisis important for, as is shown in FIG. 5, if the wires 41 of a fieldforming grid 43 were located parallel to the major axis, i.e.,vertically, a slight amount of defocusing would then be encountered inthe X-Y plane which would detract from the major focusing effect due tothe curvature of the grid itself. With conductors arranged transversely,the grid is also subject to the minor defocusing effect: however, theperformance of the system is not impaired, since the defocusing is inthe X-Z plane, and hence does not detract from the lens effect in theX-Y plane.

Still another alternative is to provide a mesh-type array as the secondelectrode and provide it with curvature in two planes as above. Whileyielding a major focusing effect on both the major and minor axis ofslit 38, such a configuration is subject to the same slight disadvantagedue to the deformation of the electric field adjacent the wires of themesh electrode as indicated above. As shown in conjunction with FIG. 5spaced apart conductors contribute minor defocusing effects opposing themajor effect. A mesh is also subject to decreased electricaltransparency since more ions traveling between the first and thirdelectrodes would encounter the conductors of the second electrode. Thisreduces the ion yield in the analyzing sector.

Multiple grids, as shown in FIG. 4, are provided in the inter-electrodespace between the first aperture 37 and object slit 39 where it isdesired to form a more predictable field and to minimize fielddistortion due to surface charges or nearby conductors. These electrodesare designated 56 and 58. For focusing purposes the curvature of theextra electrodes 56 and 5S is also selected such that their center offocus is located at the center of the slit 38. In the embodiment shownin FIG. 4 electrodes 56 and 5S are circular segments as is electrode 34.In addition, the electrodes S6 and 53 are connected to the high voltagesupply at 60 and 62 such that their potential is intermediate that ofthe first aperture 37 and the object slit 39. To achieve greatest ionyields, the electrodes in this configuration are arranged such thatcorresponding conductors in each electrode are located in the sameplane. This means that corresponding conductors in adjacent electrodesare located in the electrical shadow of each other so that the effectiveelectrical transparency of the combination is the same as if only onefield forming grid were intcrpocd in this interclccfrode space.

Although the electrode system of this invention has been described inconjunction with an ion source for use with mass spectrometer and inparticular a solids mass spectrometer using a spark source, such anillustration is intended to be used by way of example only. Theelectrode system of this invention is capable of being used with anytype of ion source in the various mass spectrometers or analyzers whichare common in the art.

I claim:

1. 1n a rnass analyzer the combination comprising:

an ionization chamber,

eans for producing ions within the chamber,

a first electrode with an aperture provided therein located adjacent theion producing means,

a third electrode with an elongated vertical slit provided thereinlocated on the side of the first electrode oppoiste the ion producingmeans, the slit being aligned with the aperture in the first electrode,

a second electrode mounted on the third electrode on the side thereofadjacent the first electrode, the second electrode comprising aframework of horizontal curved, parallel conductors, each of saidconductors being arranged traveresly with respect to the vertical slitin the third electrode and in a convex relation with respect to thefirst electrode and a concave relation with respect to the thirdelectrode, and

means for imposing a potential difference between the first and secondelectrodes whereby an electric field is created between the first andsecond electrodes, and ions passing through the aperture in the firstelectrode are accelerated by the difference of potential between thefirst and second electrodes and focused on the slit in the thirdelectrode by means of the electric field.

2. In a mass spectrometer, the combination comprising:

an ionization chamber,

means for producing ions within the chamber,

a first electrode with a circular aperture provided therein locatedadjacent the ion producing means,

a first source of potential connected to the first electrode,

a third electrode with a vertical slit provided therein having a majorand a minor axis defining one boundary of the ionization chamber, theslit being aligned with the aperture in the first electrode,

a second electrode mounted yon the third electrode on the side adjacentthe first electrode, the second electrode including a framework ofspaced, horizontal parallel conductors aligned with the minor axis ofthe slit in the third electrode, each of the conductors forming circularsegments with centers coinciding with the major axis of the thirdelectrode slit such that each `of said conductors in said framework iscurved convexly in relation to the first electrode and concavely inrelation to the third electrode, and a second source of potentialconnected to the second and third electrodes, the second source beinglower in magnitude than the first whereby ions passing through theaperture in the first electrode are accelerated by the difference ofpotential between the first and second electrodes and are focused on themajor axis of the slit in the third electrode by means of an electricfield existing between the first and second electrodes due to thedifference of potential between the two electrodes.

3. In a mass spectrometer, the combination comprising:

an ionization chamber,

a spark source of positive ions located within the chamber,

a radio frequency source connected to the spark source,

a first electrode with a circular aperture provided therein locatedadjacent the spark source and electrically connected to the sparksource,

a first source of potential connected to the spark source and the rstelectrode,

a third electrode located on the side of the first electrode oppositethe spark source and aligned with the first electrode, the thirdelectrode including an elongated vertical slit having a major and aminor axls,

a second electrode mounted on the third electrode on the side adjacentthe first electrode and electrically connected to the third electrode,the second electrode including a framework of spaced, arcuate, parallelwires aligned with the minor axis of the elongated slit in the thirdelectrode, the center of curvature of the wires coinciding with themajor axis of the third electrode slit such that each of said conductorsin said framework is curved convexly in relation to the first electrodeand concavely in relation to the third electrode, and

a second source of potential connected to the second and thirdelectrodes, the second source being lower in magnitude than the first,whereby the ions created by the spark source diffuse through theaperture in the first electrode, are accelerated by the difference ofpotential between the first and second electrodes and are focused on themajor axis of the elongated slit in the third electrode by means of anelectric field existing between the first and second electrodes due tothe difference of potential established therebetween.

4. The combination according to claim 3 wherein at least one additionalelectrode is located in the space between the first and thirdelectrodes, said electrode being congruent with the second electrode andcomprising a framework of spaced, arcuate, parallel wires aligned withthe minor axis of the slit in the third electrode and with the wires ofthe second electrode, the center of curvature of the wires of saidadditional electrode coinciding with the major axis of the thirdelectrode slit such that each of said w-ires in said additionalelectrode is convex with respect to the first electrode and concave withrespect to the third electrode.

References Cited UNiTED STATES PATENTS 3/1957 Lawrence z Z50-41.9 9/1958Robinson Z50-41.9

1. IN A MASS ANALYZER THE COMBINATION COMPRISING: AN IONIZATION CHAMBER,MEANS FOR PRODUCING IONS WITHIN THE CHAMBER, A FIRST ELECTRODE WITH ANAPERTURE PROVIDED THEREIN LOCATED ADJACENT THE ION PRODUCING MEANS, ATHIRD ELECTRODE WITH AN ELONGATED VERTICAL SLIT PROVIDED THEREIN LOCATEDON THE SIDE OF THE FIRST ELECTRODE OPPOSITE THE ION PRODUCING MEANS, THESLIT BEING ALIGNED WITH THE APERTURE IN THE FIRST ELECTRODE, A SECONDELECTRODE MOUNTED ON THE THIRD ELECTRODE ON THE SIDE THEREOF ADJACENTTHE FIRST ELECTRODE, THE SECOND ELECTRODE COMPRISING A FRAMEWORK OFHORIZONTAL CURVED, PARALLEL CONDUCTORS, EACH OF SAID CONDUCTORS BEINGARRANGED TRAVERESLY WITH RESPECT TO THE VERTICAL SLIT IN THE THIRDELECTRODE AND IN A CONVEX RELATION WITH RESPECT TO THE FIRST ELECTRODEAND A CONCAVE RELATION WITH RESPECT TO THE THIRD ELECTRODE, AND MEANSFOR IMPOSING A POTENTIAL DIFFERENCE BETWEEN THE FIRST AND SECONDELECTRODES WHEREBY AN ELECTRIC FIELD IS CREATED BETWEEN THE FIRST ANDSECOND ELECTRODES, AND IONS PASSING THROUGH THE APERTURE IN THE FIRSTELECTRODE ARE ACCELERATED BY THE DIFFERENCE OF POTENTIAL BETWEEN THEFIRST AND SECOND ELECTRODES AND FOCUSED ON THE SLIT IN THE THIRDELECTRODE BY MEANS OF THE ELECTRIC FIELD.